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Contents
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5-1
Introduction
Types and Variables
Statements and Control Flow
Reading Input
Classes and Objects
Arrays
Methods
Scope and Lifetime
Utility classes
Introduction to Object-Oriented Analysis
and Design
Copyright (c) 1999-2004 N. Afshartous
Chapter 5: Classes and Objects
After this chapter you will be able to:
-
5-2
Declare object variables
Create objects
Incorporate String objects
Use different String operations
Copyright (c) 1999-2004 N. Afshartous
Object-Oriented Terms
• Classes specify what operations an object
supports
• A Car class might define start and stop
• An object is an instance of some class
corvette
class Car
taurus
Objects
Objects
jeep
5-3
Copyright (c) 1999-2004 N. Afshartous
Another Example
• Class Person specifies the behavior of a
person
fred
class
Person
sally
Objects
Objects
rachel
• Fred, Sally and Rachel are objects of type
Person
5-4
Copyright (c) 1999-2004 N. Afshartous
Memory
• Memory is sequence of cells which can be
identified using addresses or names (program
variables)
Addresses
0
1
2
Names
4
x
int x = 4;
3
5-5
Copyright (c) 1999-2004 N. Afshartous
Declaring Object Variables
• Once a class is defined, the class name can be
used as a type in a variable declaration
• A predefined String class exists in Java
String x;
• x is a variable that can store the memory
address of a String object
• We will only be instantiating existing classes
in this course
5-6
Copyright (c) 1999-2004 N. Afshartous
Creating Objects
• The new operator creates objects
• Syntax: new ClassConstructor(args)
new String(“hello");
• The class constructor is a special operation
provided by a class
• A class constructor always has the same
name as its class
5-7
Copyright (c) 1999-2004 N. Afshartous
Initializing String Variables
• An object can be assigned to an object
variable
String x = new String(“hello");
• x stores the address of a String object
x
5-8
“hello"
Copyright (c) 1999-2004 N. Afshartous
String Init Shorthand
• Either the long or short form may be used
5-9
String x = new String(“hello");
x
“hello"
String y = “hello";
y
“hello"
Copyright (c) 1999-2004 N. Afshartous
String Assignment
• A new string may be subsequently assigned to
a string variable
String x = “hello";
x
“hello"
x = “there";
The object storing “hello" is no
longer accessible and will be
garbage collected
x
5-10
“hello"
“there"
Copyright (c) 1999-2004 N. Afshartous
String Assignment
• Assignment between object variables copies
addresses
x
“hello"
String x = “hello", y;
y
y = x;
• x and y store the same
address
5-11
x
“hello"
y
Copyright (c) 1999-2004 N. Afshartous
Assignment with Primitive Types
• Assignment involving variables having
primitive types copies values
int x = 44, y=33;
x 44
y 33
x 33
y 33
x = y;
Object variables store addresses, variables of
primitive type store values
5-12
Copyright (c) 1999-2004 N. Afshartous
String Concatenation
• The plus operator (+) concatenates strings
String x = “hello";
String y = “ there", z;
int a = 4;
x
“hello"
y
“ there"
z
“hello there"
z = x + y;
System.out.println(“z = “ + z);
System.out.println(“a = “ + a);
Output
Output
5-13
z = hello there
a=4
Copyright (c) 1999-2004 N. Afshartous
Methods
• A method is a procedure defined in a class
• The String class defines the charAt method
• Signature:
char charAt (int)
return type
method
name
argument
type
• Lookup charAt under class String in API doc
5-14
Copyright (c) 1999-2004 N. Afshartous
Using charAt
• charAt is used to extract individual
characters from a String
• Indexing starts from 0
String x = “hello";
System.out.println(x.charAt(0));
System.out.println(x.charAt(1));
Output
Output
5-15
h
e
Copyright (c) 1999-2004 N. Afshartous
String Lengths
• Can find out the length of any String using
Var.length()
String x = “hello";
for (int i = 0; i < x.length(); i++)
System.out.println(x.charAt(i));
Output
Output
5-16
h
e
l
l
o
Copyright (c) 1999-2004 N. Afshartous
Printing A String In Reverse
String x = “Java Mania!";
for (int i = x.length() - 1; i >= 0; i--)
System.out.print(x.charAt(i));
System.out.println();
Output
Output
5-17
!ainaM avaJ
Copyright (c) 1999-2004 N. Afshartous
Printing A String In Reverse
(Another Way)
String x = “Java Mania!";
String y = ““;
for (int i = x.length() - 1; i >= 0; i--)
y += x.charAt(i);
System.out.println(y);
Output
Output
5-18
!ainaM avaJ
Copyright (c) 1999-2004 N. Afshartous
Comparing Strings
• The equal operator (==) when comparing
objects just compares addresses
String x = “Java";
String y = x;
String z = “Java";
x
if (x == y)
System.out.println(“x == y");
y
if (x != z)
System.out.println(“x != z");
Output
Output
5-19
z
“Java"
“Java"
x == y
x != z
Copyright (c) 1999-2004 N. Afshartous
Equals Method
• The String class has an equals method that
compares the String values (not addresses)
String x = “Java";
String y = x;
String z = “Java";
if (x.equals(z))
System.out.println(“x equals z);
if (x != z)
System.out.println(“x != z");
Output
Output
5-20
x equals z
x != z
Copyright (c) 1999-2004 N. Afshartous
Upper and Lower Case
Conversion
String x = “Java";
String y = “ Rules";
String a = x.toUpperCase();
String b = y.toLowerCase();
System.out.println(x + y);
System.out.println(a + b);
• Output ?
5-21
Copyright (c) 1999-2004 N. Afshartous
String Methods
• public int compareTo(String
anotherString)
• public String concat(String
str)
• public String substring(int
beginIndex)
• public String toLowerCase()
• public String toUpperCase()
• public boolean startsWith(String
5-22
prefix)
Copyright (c) 1999-2004 N. Afshartous
It’s Exercise Time
5-23
Copyright (c) 1999-2004 N. Afshartous